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    <meta charset="UTF-8">
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    <title>Document</title>
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<body>
    <script>
        arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13]
        function getMissingNum(arr) {
            var n = arr.length + 1;
            var Sum1 = n * (n + 1) / 2;
            var Sum2 = 0;
            arr.map(item => Sum2 += item);
            return Sum1 - Sum2;
        }
        console.log(getMissingNum(arr));
    </script>
</body>

</html>